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4.5Inertial and Noninertial Frames of Reference
One day, you’re driving down the street in your pickup truck, on your
way to deliver a bowling ball. The ball is in the back of the truck, enjoying
its little jaunt and taking in the fresh air and sunshine. Then you have to
slow down because a stop sign is coming up. As you brake, you glance in
your rearview mirror, and see your trusty companion accelerating toward
you. Did some mysterious force push it forward. No, it only seems that way
because you and the car are slowing down. The ball is faithfully obeying
Newton’s first law, and as it continues at constant velocity it gets ahead
relative to the slowing truck. No forces are acting on it (other than the same
canceling-out vertical forces that were always acting on it). The ball only
appeared to violate Newton’s first law because there was something wrong
with your frame of reference, which was based on the truck.
How, then, are we to tell in which frames of reference Newton’s laws are
valid. It’s no good to say that we should avoid moving frames of reference,
because there is no such thing as absolute rest or absolute motion. All
frames can be considered as being either at rest or in motion. According to
(a) In a frame of reference that moves with
the truck, the bowling ball appears to violate
Newton's first law by accelerating despite
having no horizontal forces on it.
(b) In an inertial frame of reference, which the surface of the earth
approximately is, the bowling ball obeys Newton's first law. It
moves equal distances in equal time intervals, i.e. maintains
constant velocity. In this frame of reference, it is the truck that
appears to have a change in velocity, which makes sense, since
the road is making a horizontal force on it.
Chapter 4Force and Motion